Technical Field
The present disclosure relates to a communication method. More particularly, the present disclosure relates to a LTE TDD communication method.
Description of Related Art
Time division duplex (TDD) offers flexible deployments without requiring a pair of spectrum resources. Currently, LTE (Long Term Evolution) TDD allows for asymmetric downlink-uplink (DL-UL) allocations by providing seven different semi-statically configured DL-UL configurations. These allocations can provide between 40% and 90% DL sub-frames. The semi-static allocation may or may not match the instantaneous traffic situation. The current mechanism for adapting DL-UL allocation is based on the system information change procedure. Additional mechanisms could include, for example, dynamic allocation of sub-frames to UL or DL.
Compared with the system information change procedure, the dynamic mechanisms allow a much shorter period for TDD DL-UL reconfiguration. Such an idea is termed “Further Enhancements to LTE TDD for DL-UL Interference Management and Traffic Adaptation” (eIMTA) in 3GPP. Evaluations in the corresponding study item reveal significant performance benefits by allowing TDD DL-UL reconfiguration based on traffic adaptation in small cells. Furthermore, dynamic signaling mechanisms outperform that using the system information change procedure.
Hybrid automatic repeat request (HARQ) is a transmission technique widely adopted in modem wireless communication systems. HARQ operates by retransmitting an identical copy of the original transmission or another redundancy version upon transmission error. The receiver then combines the previously corrupted transmissions with the retransmitted one.
In LTE TDD systems, the timing relation between the feedback information indicating a transmission error and the corresponding retransmission are separately and differently defined for each of the 7 configurations due to the different allocations of the DL-UL sub-frames. Furthermore, the UL HARQ processes are synchronous, which means that the timing for feedback information and the corresponding retransmissions are fixed instead of being scheduled on-the-fly. As a consequence, dynamic switching among different TDD configurations gives rise to HARQ timing mismatches. Such a mismatch problem could severely affect the transmission performance and degrade the achievable performance gain brought by the dynamic TDD traffic adaptation.
Since the transmit power of UEs (user equipments, for example, mobile devices) are significantly lower than that of the base station, UL coverage enhancement is a fundamental technology employed in LTE systems. The technique of transmission time interval (TTI) bundling bundles together a group of UL sub-frames transmitting the same physical data. The base station can then achieve decoding gain by soft combining the received UL data. Currently, the number of UL sub-frames bundled together in LTE TDD systems is 4. Note that up to 3GPP Rel-11, some of the TDD DL-UL configurations do not support TTI bundling due to the fact that there are less than 4 UL sub-frames in a radio frame. Therefore, in LTE TDD eIMTA system, the technique of TII bundling for enhancing UL coverage cannot be directly applied since the number of UL sub-frames per radio frame changes dynamically in different TDD DL-UL configurations.